Polarization recording of holograms and related “polarization gratings” were conceived in 1970's as a method for recording and reconstructing the vector field of light. A light-sensitive material that acquired birefringence under the action of polarized light was suggested in the first studies (Sh. D. Kakichashvili, “Method for phase polarization recording of holograms,” Sov. J. Quantum. Electron. 4, 795, 1974). Examples of such photoanisotropic media included colored alkaly halid crystals regarded particularly promising due to reversibility of the recording process consisting in optically altering the orientation of anisotropic color centers in the crystal. A medium possessing with photoinduced birefringence was used for polarization sensitive optical storage by Kawano et al. as disclosed in US Patent application US 2001/0002895.
A grating characterized only by spatial variations in the orientation of the induced optics axis can be obtained when the photoanisotropic medium is exposed to a constant intensity, rectilinear light vibrations, with spatially varying orientation, obtained from superposition of two orthogonal circularly polarized waves propagating in slightly different directions (M. Attia, et al., “Anisotropic gratings recorded from two circularly polarized coherent waves,” Opt. Commun. 47, 85, 1983). The use of Methyl Red azobenzene dye in a polymer layer allowed to claim that photochemical processes in such material systems would enable obtaining 100% diffraction efficiency even in “thin” gratings (T. Todorov, et al., “High-sensitivity material with reversible photo-induced anisotropy,” Opt. Commun. 47, 123, 1983). Highly stable polarization gratings with orthogonal circular polarized beams are obtained in thin solid crystalline Langmuir-Blodgett films composed of amphiphilic azo-dye molecules showing that “100% efficiency may be achieved for samples less than 1 μm thick” (G. Cipparrone, et al., “Permanent polarization gratings in photosensitive langmuir-blodget films,” Appl. Phys. Lett. 77, 2106, 2000).
A material possesing birefringence that is not influenced by light is an alternative to the photoanisotropic materials that are typically capable of only small induced birefringence (L. Nikolova et al., “Diffraction efficiency and selectivity of polarization holographic recording,” Optica Acta 31, 579, 1984). The orientation of such a material, a liquid crystal (LC), can be controlled with the aid of “command surfaces” due to exposure of the substrate carrying the command layer to light beams (K. Ichirnura, et al., “Reversible Change in Alignment Mode of Nematic Liquid Crystals Regulated Photochemically by Command Surfaces Modified with an Azobenzene Monolayer,” Langmuir 4, 1214, 1988). Further a “mechanism for liquid-crystal alignment that uses polarized laser light” was revealed (W. M. Gibbons, et al., “Surface-mediated alignment of nematic liquid crystals with polarized laser light,” Nature 351, 49, 1991; W. M. Gibbons, et al., “Optically controlled alignment of liquid crystals: devices and applications,” Mol. Cryst. Liquid. Cryst., 251, 191, 1994). Due to localization of dye near the interface, the exposure can be performed in the absence of LC, and the LC is aligned with high spatial and angular resolution (potentially, submicron) after filling the cell (W. M. Gibbons, et al., “Optically generated liquid crystal gratings,” Appl. Phys. Lett. 65, 2542, 1994). Variety of photoalignment materials are developed for achieving high-resolution patterns and obtaining variation of molecular alignment within individual pixels (M. Schadt, et al., “Optical patterning of multi-domain liquid-crystal displays with wide viewing angles,” Nature 381, 212, 1996).
A critically important issue for producing LC orientation patterns at high spatial frequencies is their mechanical stability. Particularly, the cycloidal orientation of LCs obtained due to the orienting effect of boundaries is stable only when a specific condition between the material parameters, the cell thickness, and the period of LC orientation modulation is fulfilled (H. Sarkissian et al., “Periodically Aligned Liquid Crystal: Potential application for projection displays,” Storming Media Report, A000824, 2004; H. Sarkissian, et al., “Periodically aligned liquid crystal: potential application for projection displays and stability of LC configuration,” Optics in the Southeast 2003, Orlando, Fla.; Conference Program, PSE 02. and H. Sarkissian, et al., “Potential application of periodically aligned liquid crystal cell for projection displays,” Proc. of CLEO/QELS Baltimore Md., poster JThE12, 2005; B. Ya. Zeldovich, N. V. Tabirian, “Devices for displaying visual information,” Disclosure, School of Optics/CREOL, July 2000). Suggesting fabrication of cycloidal polarization gratings using the photoalignment technique with overlapping right and left circularly polarized beams, the publications by Sarkissian, Zeldovich and Tabirian cited above are credited for having theoretically proven polarization gratings can be 100% efficient and can be used as a diffractive grating for projection displays (C. Provenzano, et al., “Highly efficient liquid crystal based diffraction grating induced by polarization holograms at the aligning surfaces,” Appl. Phys. Lett., 89, 121105, 2006; M. J. Escuti et al., “A polarization-independent liquid crystal spatial-light-modulator,” Proc. SPIE 6332, 63320M, 2006).
LCs with spatially modulated orientation patterns produced using the photoalignment technique are known in the prior art (W. M. Gibbons, et al., “Surface-mediated alignment of nematic liquid crystals with polarized laser light,” Nature 351, 49, 1991; C. M. Titus et al., “Efficient, polarization-independent, reflective liquid crystal phase grating,” Appl. Phys. Lett. 71, 2239, 1997; J. Chen, et al., “An electro-optically controlled liquid crystal diffraction grating, Appl. Phys. Lett. 67, 2588, 1995; B. J. Kim, et al., “Unusual characteristics of diffraction gratings in a liquid crystal cell,” Adv. Materials 14, 983, 2002; R.-P. Pan, et al., “Surface topography and alignment effects in UV-modified polyimide films with micron size patterns,” Chinese J. of Physics 41, 177, 2003; A. Y.-G. Fuh, et al., “Dynamic studies of holographic gratings in dye-doped liquid-crystal films,” Opt. Lett. 26, 1767, 2001; C.-J. Yu, et al., “Polarization grating of photoaligned liquid crystals with oppositely twisted domain structures,” Mol. Cryst. Liq. Cryst. 433, 175, 2005; G. Crawford, et al., “Liquid-crystal diffraction gratings using polarization holography alignment techniques,” J. of Appl. Phys. 98, 123102, 2005; Crawford et al., U.S. Pat. No. 7,196,758).
LC polymers were widely used as well (M. Schadt, et al. “Photo-Induced Alignment and Patterning of Hybrid Liquid Crystalline Polymer Films 011 Single Substrates,” Jpn. J. Appl. Phys. 34, L764 1995; M. Schadt, et al. “Photo-Generation of Linearly Polymerized Liquid Crystal Aligning Layers Comprising Novel, Integrated Optically Patterned Retarders and Color Filters,” Jpn. J. Appl. Phys. 34, 3240, 1995; Escutti et al, US Patent Application US2008/0278675). Photo-aligned anisotropic thin films can be applied to rigid or flexible substrates, which may be flat or curved and/or generate patterned retarders with continuous or periodical inplane variation of the optical axis (H. Seiberle, et al., “Photo-aligned anisotropic optical thin films,” SID 03 Digest, 1162, 2003).
The CDWs wherein the optical axis of the material is periodically rotating in the plane of the waveplate along one axis of a Cartesian coordinate system are among the most interesting one-dimensional structures used for applications such as displays, beam steering systems, spectroscopy etc. These are known also as optical axis gratings, and polarization gratings (PGs) (S. R. Nersisyan, et al., “Optical Axis Gratings in Liquid Crystals and their use for Polarization insensitive optical switching,” J. Nonlinear Opt. Phys. & Mat. 18, 1, 2009). Some interesting for applications two-dimensional orientation patterns possess with axial symmetry (N. V. Tabiryan, et al., “The Promise of Diffractive Waveplates,” Optics and Photonics News 21, 41, 2010; L. Marucci, US Patent Application 2009/0141216; Shemo et al., US Patent Application 2010/0066929) and may have nonlinear dependence on coordinates.
It is important to introduce a clear distinction between polarization holograms, polarization gratings and diffractive waveplates as referenced to in further discussion. Polarization holograms are recorded with overlapping orthogonal polarized reference and pump beams in photoresponsive anisotropic materials. It is generally implied that the reference beam is spatially modulated and carries information.
The term “polarization grating” usually refers to a polarization hologram recorded with two orthogonal polarized beams that are not spatially modulated to carry information. Typically, these beams are equal in intensity and none can be singled out as reference.
Waveplates are thin anisotropic films with special conditions on orientation of optical axis and phase retardation L(ne−n0)=λ/2, where ne and n0 are the principal values of refractive indices of the material, and L is the thickness, The optical axis orientation is spatially modulated in DWs. Particularly, CDWs present rotation of the optical axis of the material at a constant rate along a single axis of a Cartesian coordinate system. To the degree CDWs can be produced using holography techniques, they can be regarded as a subclass of polarization holograph y or polarization gratings.
Axial DWs (ADWs or vortx waveplates) are the polar analog of CDWs and cannot be obtained with polarization holography techniques. As mentioned above, DWs can possess with more complex optical axis modulation patterns, and they are distinguished with a specific phase retardation condition.
Thus, in the prior art, optical axis modulation patterns of anisotropic material systems were demonstrated, including in LCs and LC polymers, due to modulation of boundary alignment conditions, and it was shown that such boundary conditions can be achieved by a number of ways, including using photoaligning materials, orthogonal circular polarized beams, microrubbing, and substrate rotation (Fünfshilling et al., U.S. Pat. No. 5,903,330; B. Wen, et al., “Nematic liquid-crystal polarization gratings by modification of surface alignment,” Appl. Opt. 41, 1246, 2002; S. C. McEldowney et al., “Creating vortex retarders using photoaligned LC polymers,” Opt. Lett., Vol. 33, 134, 2008). LC optical components with orientation pattern created by exposure of an alignment layer to a linear polarized light through a mask, by scanning a linear polarized light beam in a pattern, or creating a pattern using an interference of coherent beams is disclosed in the U.S. Pat. No. 5,032,009 to Gibbons, et al. Also, in the prior art, “Optically controlled planar orientation of liquid crystal molecules with polarized light is used to make phase gratings in liquid crystal media” (W. M. Gibbons and S.-T. Sun, “Optically generated liquid crystal gratings,” Appl. Phys. Lett. 65, 2542, 1994).
DWs are characterized by their efficiency, optical homogeneity, scattering losses, and size. While acceptable for research and development purposes, none of the techniques known in the prior art can be used for fabricating high quality DWs and their arrays in large area, inexpensively, and in high volume production. Since DWs consist of a pattern of optical axis orientation, they cannot be reproduced with conventional techniques used for gratings of surface profiles (J. Anagnostis, D. Rowe, “Replication produces holographic optics in volumes”, Laser Focus World 36, 107, 2000; M. T. Gale, “Replicated diffractive optics and micro-optics”, Optics and Photonics News, August 2003, p. 24).
It is the purpose of the present invention to provide method for the production of DWs of continuous patterns of optical axis orientation and controlling their spatial period. The printing method of the current invention does not require complex holographic setups, nor special alignment or vibration isolation as described in the publications S. R. Nersisyan, et al., “Optical Axis Gratings in Liquid Crystals and their use for Polarization insensitive optical switching,” J. Nonlinear Opt. Phys. & Mat., 18, 1, 2009; S. R. Nersisyan, et al., “Characterization of optically imprinted polarization gratings,” Appl. Optics 48, 4062, 2009 and N. V. Tabiryan, et al., “The Promise of Diffractive Waveplates,” Optics and Photonics News, 21, 41, 2010, which are incorporated herein by reference.
Energy densities required for printing DWs are essentially the same as in the case of producing a waveplate in a holographic process. This makes fabrication of diffractive waveplates much faster compared to mechanical scanning or rotating techniques. A technique for obtaining polarization modulation patterns avoiding holographic setups was discussed earlier in the U.S. Pat. No. 3,897,136 to 0. Bryngdahl. It discloses a grating “formed from strips cut in different directions out of linearly dichroic polarizer sheets. The gratings were assembled so that between successive strips a constant amount of rotation of the transmittance axes occurred.” These were also essentially discontinuous structures, with the angle between the strips π/2 and π/6 at the best. The size of individual strips was as large as 2 mm. Thus, such a grating modulated polarization of the output light at macroscopic scales and could not be used for production of microscale-period gratings with diffractive properties at optical wavelengths.